This wondrous song, called 'Night, Night, Little Marvin', is the one Marshall and Lily use to wish a goodnight to their son, Marvin Waitforit Eriksen, who was born in 2012, just at the end of season 7 -Magician's Code chapters 1 and 2-.
Jason Segel's voice is so sweet and tender, and Lily looks so proffessional when it comes to instrumental skills, so I gotta say they make a wonderful couple not just as husband and wife, but as musicians too.
For those interested, the song appears on the sitcom's episode 'The Final Page Part 2', season 8, the same episode when Ted inaugurates his greatest artwork to date, the GNB Tower, and when Barney proposes marriage to Robin in the roof of WWN Building -to me, one of the sitcom's best episodes ever!-. And now, with no further delay, here you are the one and only 'Night, Night, Little Marvin' as performed by his dedicated parents, Marshall Eriksen and Lily Aldrin! :D
The original video, as appears on the already mentioned episode :)
The official song, full version :)